1. Field of the Invention
This invention pertains generally to gas turbine engine driven powerplants, and more particularly to a gas turbine driven powerplant in which a heat exchanger is used to simultaneously cool high temperature compressor discharge air and heat and/or reform a fuel and water mixture for the combustor. The cooled compressed air exiting the heat exchanger provides ultra-cool air for cooling metal components, for recuperating normally rejected low grade heat from other powerplant ancillary operations, and for feeding through a plate-fin regenerator to recover and return power turbine exhaust heat to the combustor. The hot fuel mixture exiting the heat exchanger may be sent directly to the combustor or first chemically reformed to produce a low pollutant producing fuel gas. Alternatively, instead of feeding the bulk of the cooled compressed air through a plate-fin recuperator, the cool air from the exchanger can be routed directly to the combustor. Instead of recuperating the exhaust heat with cool air, an additional fuel/water mix will recuperate the exhaust heat. The resulting superheated fuel gas increases mass flow through the turbine proportionally increasing power output.
2. Description of the Background Art
Gas turbine engines are in wide use, and are ever more often the prime mover of choice. For example, the jet engine is an example of a successful gas turbine application since gas turbine engines have no equal for powering large aircraft. While the modern jet engine is the product of over 50 years of engineering development, the jet engine must fly and, therefore, the design options to enhance performance are necessarily limited. On the other hand, ground applications of advanced aircraft gas turbine engines allow use of additional performance enhancing techniques. In the air or on the ground, however, the principal long term route to increased powerplant performance has been through higher engine compression ratios and higher firing temperatures.
Higher firing temperatures have evolved through a succession of innovative cooling strategies involving compressor bleed air used as turbine coolant to maintain acceptable limits on sustainable temperatures seen by the turbine's metal alloys. Bleed air flow, however, reduces gas turbine power and performance. The ever higher compression ratios also necessarily mean higher compressor discharge air temperatures, thereby limiting bleed air cooling effectiveness and requiring additional bleed air flows of the high temperature, high pressure air to effect the same metal cooling. The present invention, as will be shown, will enable metal component cooling with compressor discharge air at or near ambient temperatures. The new coolant, it will be shown, will allow for a significant reduction in bleed air required and/or a significant increase in firing temperature and/or both. The new coolant will also allow creation of a low temperature heat sink. Low grade heat is normally too cool to be recovered and recycled and, therefore, generally requires continuous removal and rejection to the surroundings by means of a cooling tower or the like. Such heat is generated during operation of ancillary powerplant equipment from sources such as friction in the bearings of the generator and of the gas turbine, and appears in the hot lubricating oil. Low grade heat is also produced by the generator windings in the form of "copper losses," amounting to as much as 0.5 to 1.5 percent of the electrical energy generated, and is generally continuously removed by means of a water cooling loop. Also, the electrical transformer experiences "iron losses" due to the hysteresis effect in the iron core of the transformer. This significant heat load is carried away in the circulating transformer oil, and the heat in the hot oil is rejected to the atmosphere by means of air blast heat exchangers. Economic recovery of this low grade heat for return to the cycle is not possible without a low temperature heat sink which is allowed by this invention.
In ground applications, the recovery, recycle, and conversion of heat remaining in the gas turbine exhaust has been another route to higher overall powerplant performance. The high temperature turbine exhaust heat is normally recovered by conventional steam raising techniques, or, in older low pressure gas turbine powerplants, by a regenerator (plate-fin air-to-gas heat exchanger) employing a counter current discharge air against the flow of higher temperature turbine exhaust gas. The ever increasing compressor discharge air temperature of the newer gas turbines, however, limits the heat recovery effectiveness of regenerators. In gas turbines derived from advanced aircraft engines, the compressor discharge temperatures are 200 to 300 degrees Fahrenheit greater than turbine exhaust temperature, thereby ruling out use of a regenerator altogether. In the high pressure turbine engine driven powerplant, steam is raised in a conventional boiler to extract exhaust heat, routed to a condensing steam turbine with condensate recycled to the boiler. This combination of a gas turbine and steam turbine bottoming cycle is referred to as a combined cycle powerplant. The present invention, as will be shown, capitalizes on the higher compressor discharge air temperatures in the advanced aircraft-engine-derived gas turbines and, in doing so, eliminates the relatively unproductive steam cycle from the powerplant.
Alternatively, heat recuperating steam raised in the boiler from the turbine exhaust heat is simply injected into the gas turbine flow path before the combustor for power augmentation. The gas turbine engine's increasingly higher firing temperatures and compression ratios thermodynamically favor the steam injection option over the steam bottoming cycle. Additionally, direct steam injection into the gas turbine combustor obviates the need for purchase and operation of a condensing steam turbine, condenser, cooling tower, and interconnecting circulating water piping. Elimination of the steam bottoming cycle and its associated equipment increases overall plant durability, reliability, availability, and maintainability. In the present invention, as will be shown, the recuperated heat is returned to the gas turbine combustor as high energy steam with all of the previously stated performance advantages of steam injection over steam cycle accruing.
As firing temperatures are increased in advanced gas turbine engines, they produce nitrogen oxides, NO.sub.x, at exponentially increasing rates. Controlling emissions at the combustor to ever more stringent air quality emission limitations is a major combustor development problem. Peak temperatures in the combustor occur at the flame front where fuel and air react. Dilution of the fuel prior to combustion limits the flame temperatures. Fuel dilution can be effected with air, steam, or any noncombustible gas. Steam or water injection, in addition to achieving the aforementioned power augmentation, has long been used to reduce NO.sub.x formation in the combustor. Dilution can be accomplished in the combustor, but is more effective if accomplished in the fuel stream alone prior to injection into the combustor due to improved mixing. Fuel dilution with air as the diluent is currently being accomplished in specially designed premix "dry low NO.sub.x " or "dry low emission" combustors. However, with natural gas, or with liquid fuels, combustibility problems limit the amount of fuel dilution that can be accomplished before combustor flame out occurs. Several of the gas turbine manufacturers warranties are invalid if operated at fuel dilution levels of greater than two pounds of steam per pound of fuel. The NO.sub.x concentration at this flame out dilution is still many times greater than is legal to operate in California and other jurisdictions. Carbon monoxide, formaldehyde, and other unburned products also increase greatly with steam usage as combustion becomes more incomplete. NO.sub.x can be further reduced in the exhaust gas flow by injecting ammonia, together with placement of a suitable catalyst. Carbon monoxide in the exhaust gas can also be catalytically oxidized with a suitable catalyst. Emission control by means of selective catalytic reduction (SCR) with ammonia is a significant cost and liability risk as ammonia is expensive, a hazardous toxic material, and handling presents an on-going safety problem. With all of the acknowledged drawbacks SCR is (without a better option) deemed the "Best Available Control Technology" (BACT) and is legally mandated in some jurisdictions. The present invention, as will be shown, will achieve the required emission levels of all potential pollutants without the use of SCR or oxidation catalyst.
Two additional, long-recognized, design techniques for enhancing gas turbine engine performance are intercooled compression and a reheat combustor firing before the power turbine. Intercooling, while enhancing performance, also results in increased design complexity and expense for development work. Reheat, while increasing power output, also raises temperatures throughout the power turbine, requiring a major bleed of cooling air significantly reducing the net efficiency gains. Conventional reheat development costs may exceed those for intercooling. The present invention, as will be shown, does not entail the development cost associated with either intercooling or conventional reheat. Furthermore, the present invention introduces a novel method of achieving reheat with little modification required of the existing gas turbine engine.
Therefore, there is a need for a high efficiency gas turbine driven powerplant which produces low NO.sub.x emissions at higher firing temperatures. The present invention satisfies that and other needs as described herein, and overcomes the deficiencies in conventional designs.